1. Field of the Invention
The present invention relates to a light emitting diode and, more particularly, to a high-brightness light emitting diode including a reflective layer and an adhesive layer.
2. Description of Related Art
Currently, a trend of developing the light emitting diodes is to promote the brightness. In order to achieve this object, one or more reflective metal layers are combined therein. However, this metal layer has to also possess properties of adhesion and ohmic contact.
For example, R.O.C. Patent No. 369731 disclosed an LED in which the GaAs substrate is replaced with an Si substrate having a reflective metal layer thereon by wafer bonding technology. Unfortunately, such design cannot improve the lighting effect of short wavelengths.
Additionally, in U.S. Pat. No. 5,376,580, the GaAs substrate is a temporary substrate for epitaxying and then removed after being bonded to a transparent substrate. Though this method prevents absorption of the GaAs substrate, the processes have to be carried out at high temperature which might damage the structure and thus impact the lighting effect. Additionally, this transparent substrate is made by GaP which can absorb the short-wavelength light.
R.O.C. Patent No. 415116 mentioned a light emitting diode 10 as shown in FIG. 1, in which two reflective adhesive metal layers 16, 12 are respectively attached on the bottom surface of the LED epitaxial layer 15 and the top surface of the substrate 11. By bonding the two reflective adhesive metal layers 16, 12, the light beams can be propagated from the front side of the LED 10 and the brightness can be promoted.
FIG. 2 shows another conventional LED 20, in which a metal adhesive layer 22 is formed on a top surface of the SiO2 substrate 21, and an LED epitaxial layer 25 is bonded to the top surface of the metal adhesive layer 22. Brightness of the LED 20 can be improved due to ohmic contact between the substrate 21 and the metal adhesive layer 22.
For the above LED structures, all of the reflective layers have a further function of adhesion and ohmic contact, therefore only metal material is suitable. A disadvantage of such material is that diffusion occurs at the interface of the metal layer and the LED epitaxial layer. Consequently, when a light source of 600 nm or more is applied, the lighting effect will be reduced since total reflection in the metal layer is not available. FIG. 3 shows the reflectivity varied with wavelengths, in which the reflectivity is 0.9 at 600 nm. Further, the reflectivity rapidly reduces at wavelength less than 600 nm, for example, 590 nm or 570 nm of yellow-green light. Consequently, the reflective layer cannot perform expected effect. Both the reflective metal layers as shown in FIGS. 1 and 2 exist such problem.
Therefore, it is desirable to provide an improved LED structure to promote the brightness, particularly at short wavelength.
An object of the present invention is to provide a high-brightness light emitting diode, which can exhibit superior lighting effect at long wavelength.
Another object of the present invention is to provide a high-brightness light emitting diode, which can exhibit much better lighting effect than the conventional at short wavelength.
In order to achieve the above object, the high-brightness light emitting diode includes a Si substrate, a first adhesive layer, a reflective layer, an LED epitaxial layer, a first-type ohmic contact electrode and a second-type ohmic contact electrode. The first adhesive layer is formed on the Si substrate. The reflective layer is formed on the first adhesive layer. The LED epitaxial layer is formed on the reflective layer and has a pn junction structure to form a first-type layer and a second-type layer adjacent to the reflective layer. The LED epitaxial layer also has a metal contact layer formed by partial surface of the second-type layer. The first-type ohmic contact electrode has the same type as the first-type layer and formed thereon. The second-type ohmic contact electrode has the same type as the second-type layer and formed on the metal contact layer.
The LED epitaxial layer can be a p-n structure or a n-p structure, or further include a p-type confining layer and an n-type confining layer. The first-type ohmic contact electrode can further include a transparent electrode thereon to enhance electric conduction. The reflective layer usually includes at least two materials, for example, a metal and an insulator, a high-dielectric material and a low-dielectric material, etc, wherein the insulator and the low-dielectric material are preferably adjacent to the LED epitaxial layer. The metal can be Al, Ag, Au, Pt, Pd, etc. The insulator can be Al2O3, MgF2, SiO2, TiO2, Si3N4, etc. The high-dielectric material preferably has a refractive index larger than 2.1, and the low-dielectric material has a refractive index less than 1.56. The high-dielectric material can be TiO2, CeO2, Si, etc. The low-dielectric material can be Al2O3, MgF2, SiO2, Si3N4, etc. The reflective layer can further include a second adhesive layer therebelow to reinforce attachment with the first adhesive layer. The second and the second adhesive layers can be made from metal, for example, Au, Au/Be alloy, Au/Zn alloy, Pt, Pd, Cu, Ni, In and Al. The first adhesive layer also can be made from polymer nonconductor.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.